This invention relates to imaging a region of a patient using a contrast agent.
Most methods of imaging can make use of a contrast agent of one kind or another. Typically, a contrast agent is injected into the vascular system of the patient, and circulates through the body in, say, around half a minute. An image taken of the patient then shows enhanced features relating to the contrast agent.
In the case of an X-ray image, a contrast agent such as iodine absorbs X-rays and injected iodinated compounds can be used to improve contrast in view of this.
In the case of magnetic resonance (MR) imaging, a contrast agent such gadolinium chelate affects the local magnetic field and thus the interaction between the tissue to be imaged (in particular the relaxation times of protons of water molecules) and the main magnetic field. This is because the molecules of the contrast agent, which are tumbling about, are paramagnetic.
A commonly used chelate is diethylenetriaminepentaacetic (DTPA). Another contrast agent is gadolinium (Gd) DOTA. Very small iron oxide particles are also used as a contrast agent in MR imaging.
For ultrasound, air in small bubble-like cells is used as a contrast agent.
It would be usual to take an image of a region of interest of the patient before the contrast agent was injected, and then another image after the contrast agent had circulated through the body. Features of interest could be highlighted by the second image.
In the case of angiography, the two images could be subtracted to image the blood flowing through vessels. This could be used to provide information on the functioning of various organs such as the liver, kidney and brain e.g. to ascertain if the blood brain barrier had sustained any damage from a tumour, which would be indicated by blood flow across that barrier.
The problem with MR and X-ray agents is that they are excreted relatively slowly, typically over periods of hours to a few days. This means that once an agent has been given, it is present throughout any subsequent rapidly repeated examination. Hence, where subtle changes are being studied by detecting the presence and absence of the agent, there is only one opportunity for making the observations. This is a particular drawback when studying perfusion (flow through capillaries delivering blood to tissues), because the amount of blood flowing is very small. The significance in the case of MR imaging is that only a small percentage, less than 5%, of tissue protons are intravascular.